Video Transcript
A builder would like to screw some decorative metal panels to the outside of a wooden garden shed. He has the choice between aluminum, steel, or copper screws and steel or aluminum panels. Which combination of screws and panels would result in the screws, but not the panels, corroding after a few weeks of being constructed? (A) Aluminum screws and steel panels. (B) Copper screws and aluminum panels. (C) Copper screws and steel panels. (D) Steel screws and aluminum panels. Or (E) steel screws and steel panels.
The builder will need to select materials with corrosion in mind. Corrosion is the irreversible destructive process where a metal reacts with other substances to form more stable compounds. One common example of this process is when iron is exposed to oxygen in the air. It reacts readily to form iron(III) oxide, which is commonly referred to as rust.
Applications involving metals that are damaged by corrosion can be combated through alloying, which is the process of mixing a metal with other elements to produce a new material with unique properties. For example, since iron is a very useful metal but very prone to corrosion, steel is often used instead. There are many different types of steel alloys, which are mixtures between iron and carbon and sometimes include other elements, such as chromium or nickel.
In order to predict the comparative rates of corrosion of aluminum, steel, and copper, we can use a reactivity series. A reactivity series such as this one allows us to compare which metals will react and thus corrode more quickly than others. The higher up a metal is on the series, the more reactive it is. Here are the three metals available as screw materials. For our purposes, even though a steel alloy would corrode less quickly than pure iron, we will use iron’s place on the reactivity series to find the correct answer.
Aluminum is the highest of the three screw materials. This means aluminum screws would corrode before steel or copper screws. Since copper is the least reactive of these three metals, screws made of this metal would not corrode before steel or aluminum panels. So, we can eliminate answer choices (B) and (C). We can also eliminate answer choice (E), which suggests both screws in panels be composed of steel, which would not result in a difference of corrosion time.
So, between the two remaining answer choices, the combination of screws and panels that would result in the screws, but not the panels, corroding after a few weeks would be answer choice (A), aluminum screws and steel panels.
